The present invention relates to a magnetic recording medium having a ferromagnetic metal powder layer or a ferromagnetic metal thin film on a non-magnetic support. More particularly, the present invention relates to a magnetic recording medium which is excellent in running properties, durability and shelf stability under a wide variety of environmental conditions.
Hitherto, coating type magnetic recording mediums have widely been used. This type of magnetic recording medium is produced by coating a surface of a non-magnetic support with a dispersion of a powdered magnetic material, e.g., a ferromagnetic powder of an oxide or a metal or an alloy, in an organic binder, and then drying the coated dispersion.
With the increasing demand for higher-density magnetic recording, ferromagnetic metal thin film type magnetic recording mediums, which are produced by vapor deposition, sputtering, etc. without using a binder, have been put to practical use.
However, the ferromagnetic metal thin film type magnetic recording mediums have difficulty in ensuring the required running durability, although this type of magnetic recording medium is advantageous in smoothing the surface of the magnetic layer. Further, magnetic recording mediums are used under various environmental conditions, and the information recorded thereon are usually stored for a long period of time ranging from several years to several tens of years. Accordingly, magnetic recording mediums are demanded to have running properties, durability and shelf stability under a wide variety of environmental conditions.
To meet the demand, various methods have been proposed, e.g., a method wherein a lubricant is coated on the magnetic layer, a method wherein a non-magnetic protective film is provided on the magnetic layer, and a method wherein a non-magnetic protective layer and a lubricant layer are combined together.
The proposed methods suffer, however, from some problems as stated below.
For example, a magnetic recording medium having a lubricant layer formed of a material selected from among perfluoroalkyl polyethers and derivatives thereof (see Japanese Patent Application Laid-Open (KOKAI) Nos. 60-61918 and 61-107528, U.S. Pat. Nos. 3,778,308 and 4,897,211, Japanese Patent Application Post-Examination Publication No. 60-10368, etc.) has a high coefficient of friction occurring between the same and a sliding contact member during running at low speed, and hence this magnetic recording medium is unsatisfactory for practical use. The prior art magnetic recording medium is also unsatisfactory with regard to storage properties because it is corroded by an acidic gas, e.g., sulfite gas, resulting in peeling off of the magnetic recording layer. A magnetic recording medium having a lubricant layer formed of a material selected from among fatty acids and fatty acid esters (see Japanese Patent Application Post-Examination Publication Nos. 58-28367 and 51-39082, Japanese Patent Application Laid-Open (KOKAI) Nos. 56-80828 and 61-24017, etc.) suffers from a high coefficient of friction occurring between the same and a magnetic head during high-speed running and is not satisfactory with regard to corrosion by sulfite gas or other acidic gas. To lower the coefficient of friction between the magnetic recording medium and a magnetic head or other sliding contact member, it has been proposed to employ an organic fluorine compound as a lubricant, for example, fluorinated semi-fluorocompounds (see Japanese Patent Application Laid-Open (KOKAI) No. 63-281218), Perfluoroalkylcarboxylic acid amine salts (see Japanese Patent Application Laid-Open (KOKAI) No. 64-9961) and fluoroalkylcarboxylic acid amine salts (Japanese Patent Application Laid-Open (KOKAI) No. 03-241524). However, the magnetic recording mediums employing such lubricants are also unsatisfactory with regard to the corrosion by an acidic gas, for example, sulfite gas, and the running durability deteriorates with time.
As a method wherein a non-magnetic protective layer is provided, it has been proposed to provide an inorganic protective film of a non-magnetic material, e.g., carbon, silicon oxide, etc. (see Japanese Patent Application Laid-Open (KOKAI) Nos. 63-279426 and 59-56227). With this method, running properties and durability are improved, but it is necessary in order to obtain satisfactory improvements to increase the thickness of the protective layer. However, an increase in the thickness of the protective layer results in an increase in the distance between the magnetic recording medium and a signal recording/reproducing head, causing degradation of the electromagnetic transducing characteristics, Accordingly, further improvements are demanded. There is also a demand for further improvement in the shelf stability.
It has also been proposed to form a non-magnetic inorganic protective film together with a lubricant coating layer (see Japanese Patent Application Laid-Open (KOKAI) Nos. 61-236017, 62-219314, 01-166329 and 03-224132). This method makes it possible to improve all the properties, i.e., running properties, durability and shelf stability. However, the effectiveness largely differs depending upon the kind of material used for the lubricant layer, and the required characteristics have not yet sufficiently been obtained so far. Above all, shelf stability is still insufficient and demanded to improve.
It is an object of the present invention to provide a magnetic recording medium which is excellent running properties, durability and shelf stability, particularly in shelf stability to an acidic gas, under a variety of environmental conditions.